Key points
Screw feeders and conveyors are widely used for the handling of bulk materials in situations where it is beneficial for the product to be subjected to heating or cooling, in some cases this being the prime function of the equipment.
A feature of this form of material transport is that there are a range of independent design options that influence to efficiency of heat transfer that are disproportionately in the cost of manufacture.
A degree of heat transfer can be secured very economically, but fabrication cost increases according to the form of change, so an optimum cost/effective solution requires a degree of design expertise.
‘U’ trough or circular casing
The choice of ‘U’ trough or circular casing depends on various factors, such as access for cleaning, containment and heat transfer efficiency. Similarly, the selection of heat transfer media independently rests on factors such as the availability of resources, thermal task and range of temperature change.
Options include: – ambient or chilled water, oil, hot or direct fired gas or electric. However, the thermal benefit and cost of design differences in the machine construction increases in the following generate manner, although not in similar proportions.
- Fit rotatory glands on the screw shafts for heating or cooling media.
- Continuous heavy weld both sides of the screw flights.
- Fit thicker flights
- Fit shorter pitch flights
- Use larger centre tube.
- Fit jacket to casing.
- Minimise flight tip clearance by matching casing to screw deflection.
- Select oversize screw running slower.
The use of screws to extract material from the outlet slots of heat transfer hoppers enables extra cooling by adapting the screw for heat transfer by one or more of the modifications listed.
In these circumstances there is an important additional design criterion as the overall heat transfer efficiency of the system is radically influenced by the uniformity of the discharge velocity profile along the outlet slot.
An uneven rate of discharge from different regions of the hopper results in a variation in residence time. Overall heat transfer is less efficient than when the outflow is uniform, but unfortunately uniform extraction to produce an even flow rate is rarely achieved. (1).
It is well appreciated that a screw with flights of a constant pitch will preferentially extract product from the first section of the hopper to which the screw is exposed.
Common misconception for screw conveyors
However, a common misconception is that screws which vary in pitch will give a reasonably even feed, whereas the variation in feed rate is usually massive. (Fig 1.).
This assumption probably arises from the false concept that the axial transfer capacity of the screw is fixed by the swept volume of rotating, whilst it actually varies as the geometry and surface friction of the product in the inclination of the flight, which changes from where it is adjacent to the centre shaft up to the flight outside diameter. (2).
The variable flight helix affects the efficiency of axial transfer capacity, as does the frictional drag of the centre tube, so a ‘mean effective helix angle’ of flight must be assessed for extraction calculations.
A basic impediment to uniform extraction is that the initial ‘draw’ extracted from the hopper by the first flight will extract and axially transfer its full capacity, but subsequent sections of the screw can only transfer their differential increase in transfer capacity. This impediment to even extraction is addressed by a feature of feeder design, as Ajax reg. Design No. 6038827.
Screw conveyor construction
Some design expertise is then required to exploit the relative merits of design change of flight pitch, shaft size, flight thickness and the face friction. Changes in screw construction are theoretically required for every flight, but this number may be more than halved and change doubled in effectiveness by fitting right and left-hand flights to a central outlet.
The extra reduction of over 50% of the effective outlet length follows from the requirement for a central insert to prevent direct leakage of the hopper contents.
This insert must stimulate mass flow and the final two exposed flights prior to the inlet has.to extract material from directly above and half that resting above the insert.
The dynamic shear strength of the flowing media can be exploited to some extent, especially with short pitch flight sections, to mitigate minor theoretical velocity differentials.
References
- Bates.L. “Capturing the elusive benefits of uniform extraction rom hoppers” Proc. Ind.Infom. Dec. 2019.
- Bates.L. “Entrainment pattern of Screw Feeders”. J. Eng. Ind. May 1969, 91(2) : 295-302
